How far will Trump drop?

[politicallefty]

Since the Civil War, there has been a hard floor of about 36-38% of the popular vote for both parties that has never been broken except on just two occasions (the rather abnormal elections of 1912 and 1924). Even Goldwater, running against a very popular incumbent, managed to overperform the polls by at least a few points and hold 38% of the popular vote. I think the higher end of that hard floor is Trump's floor as well.

I think the bigger question in terms of down-ballot implications Hillary's ceiling. If Trump is stuck at 39-40%, there's a big difference between her getting 48-49% versus 52-53%. The latter would be a catastrophe up and down the ballot for Republicans. In either case, I think the Senate would fall. In the former, I think the House would be in play. In the latter, gerrymandering would not save the House for Republicans.